Aqueous dispersion and coating material using the same, and photocatalytic film and product

ABSTRACT

An aqueous dispersion of an embodiment includes visible-light responsive photocatalytic composite microparticles containing tungsten oxide and zirconium oxide, and an aqueous dispersion medium in which the photocatalytic composite microparticles are dispersed. In the photocatalytic composite microparticles, a ratio of a mass of the zirconium oxide to a mass of the tungsten oxide is in a range of from 0.05% to 200%, and a D50 particle size in particle size distribution is in a range of from 20 nm to 10 μm. The aqueous dispersion has pH in a range of from 1 to 9.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior International ApplicationNo. PCT/JP2013/003468 filed on May 31, 2013, which is based upon andclaims the benefit of priority from Japanese Patent Application No.2012-126577 filed on Jun. 1, 2012; the entire contents of all of whichare incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an aqueous dispersionand a coating material using the same, and a photocatalytic film and aproduct.

BACKGROUND

A tungsten oxide thin film is widely used as a dielectric material usedin capacitors, filters, electronic devices such as semiconductor chips,and the like, as an optical element material used in opticalcommunication filters, isolators, and the like, as an electrochromicmaterial used in light control mirrors and the like, and as a gaschromicmaterial used in gas sensors and the like. It is also known thattungsten oxide functions as a visible-light responsive photocatalyticmaterial, and it is a material drawing great attention in view ofindustrial applicability. Further, tungsten oxide microparticles have anexcellent function as a visible-light responsive photocatalyst, and afilm containing the tungsten oxide microparticles are drawing attention.

A photocatalytic film using tungsten oxide is formed by, for example,applying a dispersion containing tungsten oxide microparticles on asurface of a base material of a product to which photocatalyticperformance is to be imparted. It is known an aqueous dispersion whichcontains tungsten oxide microparticles having an average primaryparticle size (D50 particle size) in a range of from 1 to 400 nm andwhose pH is in a range of from 1.5 to 6.5. According to such an aqueousdispersion, dispersibility of the tungsten oxide microparticles isenhanced and formability of a film containing the tungsten oxidemicroparticles is improved. Therefore, when the film which is formed byapplying the aqueous dispersion containing the tungsten oxidemicroparticles on the base material is used as a photocatalytic film,visible-light responsive catalytic performance of the tungsten oxidemicroparticles can be exhibited.

A conventional photocatalytic film containing tungsten oxidemicroparticles exhibits a 5% gas decomposition rate or more under anenvironment where illuminance of visible light is about 2000 lx.However, considering practicality of the photocatalytic film,decomposition performance for harmful gas such as acetaldehyde is notsufficient, which has given rise to a demand for improvement of the gasdecomposition performance. Further, being poor in gas adsorbing power,the conventional photocatalytic film has a problem that its gasdecomposition speed becomes low under an environment with a low gasconcentration. Such circumstances have given rise to a demand for anincrease of the gas decomposition performance by a visible-lightresponsive photocatalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart illustrating changes of a gas decomposition rate ofphotocatalytic films formed by using aqueous dispersions of examples anda comparative example.

DETAILED DESCRIPTION

According to one embodiment, there is provided an aqueous dispersion ofan embodiment including: visible-light responsive photocatalyticcomposite microparticles containing tungsten oxide and zirconium oxide;and an aqueous dispersion medium in which the photocatalytic compositemicroparticles are dispersed. In the photocatalytic compositemicroparticles, a ratio of a mass of the zirconium oxide to a mass ofthe tungsten oxide is in a range of from 0.05% to 200%, and a D50particle size in particle size distribution is in a range of from 20 nmto 10 μm. The aqueous dispersion has pH in a range of from 1 to 9.

Hereinafter, an aqueous dispersion of an embodiment and a coatingmaterial using the same, and a photocatalytic film and a product will bedescribed. The aqueous dispersion of the embodiment includes:visible-light responsive photocatalytic composite microparticlescontaining tungsten oxide and zirconium oxide; and an aqueous dispersionmedium in which the photocatalytic composite microparticles aredispersed. pH of the aqueous dispersion of the embodiment is in a rangeof not less than 1 nor more than 9. As the aqueous dispersion medium, atleast one selected from water and alcohol is exemplified.

In the photocatalytic composite microparticles of the aqueous dispersionof the embodiment, a ratio of a mass of the zirconium oxide to a mass ofthe tungsten oxide is in a range of not less than 0.05% nor more than200%, and a D50 particle size in particle size distribution is in arange of not less than 20 nm nor more than 10 μm. In the photocatalyticcomposite microparticles, the ratio of the mass of the zirconium oxideto the mass of the tungsten oxide is more preferably in a range of notless than 0.1% nor more than 150%.

In photocatalytic composite microparticles of an aqueous dispersion ofanother embodiment, a ratio of atomicity of zirconium to atomicity oftungsten is in a range of not less than 0.05% nor more than 400%, and aD50 particle size in particle size distribution is in a range of notless than 20 nm nor more than 10 μm. In the photocatalytic compositemicroparticles, the ratio of the atomicity of the zirconium to theatomicity of the tungsten is more preferably in a range of not less than0.1% nor more than 300%.

Tungsten oxide exhibits photocatalytic performance such as gasdecomposition under visible light irradiation. However it has been foundout that a gas decomposition speed by tungsten oxide decreases as a gasconcentration reduces from an initial gas concentration. This is thoughtto be because tungsten oxide has low decomposition performance for anintermediate generated when gas is decomposed, and tungsten oxide haslow gas adsorption power under an environment with a low gasconcentration. The present inventors have found out that, in order fortungsten oxide to have improved decomposition performance for anintermediate and improved gas adsorption power, it is effective thatzirconium oxide higher in gas adsorption power than tungsten oxide iscompounded to tungsten oxide.

The photocatalytic composite microparticles used in the aqueousdispersion of the embodiment contains the zirconium oxide whose massratio to the tungsten oxide is in the range of from 0.05 to 200%. Themass ratio of the zirconium oxide to the tungsten oxide is preferably ina range of from 0.1 to 150%, and more preferably in a range of from 10to 100%. When the mass ratio of the zirconium oxide to the tungstenoxide is less than 0.05%, gas adsorption performance of the zirconiumoxide cannot be fully exhibited, and hence it is not possible to improvephotocatalytic performance of the tungsten oxide under an environmentwith a low gas concentration and the like. When the mass ratio of thezirconium oxide to the tungsten oxide is over 200%, the content of thetungsten oxide relatively reduces and accordingly performance(photocatalytic performance) itself as the visible-light responsivephotocatalytic composite microparticles deteriorates.

Further, in the photocatalytic composite microparticles used in theembodiment, the ratio of the atomicity of the zirconium to the atomicityof the tungsten is preferably in the range of from 0.05 to 400%. Theratio of the atomicity of the zirconium to the atomicity of the tungstenis more preferably in a range of from 0.1 to 300%, and still morepreferably in a range of from 10 to 200%. When the ratio of theatomicity of the zirconium is less than 0.05%, it is not possible toimprove the photocatalytic performance of the tungsten oxide under theenvironment with a low gas concentration and the like since the gasadsorption performance of the zirconium oxide cannot be sufficientlyexhibited. When the ratio of the atomicity of the zirconium is over 40%,the content of the tungsten oxide relatively reduces and accordingly theperformance (photocatalytic performance) itself as the visible-lightresponsive photocatalytic composite microparticles deteriorates.

In the photocatalytic composite microparticles used in the embodiment, acompounding method of the tungsten oxide and the zirconium oxide is notparticularly limited. As the composite microparticles of the tungstenoxide and the zirconium oxide, various kinds of composite microparticlesare usable such as mixed microparticles of tungsten oxide microparticlesand zirconium oxide microparticles (method of mixing powders), andcomposite microparticles in which the tungsten oxide carries thezirconium oxide or composite microparticles in which the zirconium oxidecarries the tungsten oxide (carrying method). In using the carryingmethod as the compounding method of the tungsten oxide and the zirconiumoxide, an immersion method using a metal solution, or the like may beemployed.

When the zirconium oxide microparticles are used as a raw material ofthe photocatalytic composite microparticles, a shape of the zirconiumoxide microparticles is not particularly limited, but primary particlesof the zirconium oxide microparticles are preferably in a rod shape.Further, zirconium oxide sol having particles in which the rod-shapedprimary particles are aggregated is more preferable. The zirconium oxidepreferably has a monoclinic crystal structure. An existing form of thezirconium oxide in the photocatalytic composite microparticles is notparticularly limited, and they can exist in various forms. Thephotocatalytic composite microparticles can contain an element substanceof the zirconium oxide or the zirconium oxide forming a complex compoundwith the tungsten oxide. The zirconium oxide may form a complex compoundwith other metal element.

The photocatalytic composite microparticles contained in the aqueousdispersion of the embodiment has an average particles size in a range ofnot less than 20 nm nor more than 10 μm. Here, the average particle sizeof the microparticles (powder) in the specification of the presentapplication refers to the D50 particle size in the particle sizedistribution. The aqueous dispersion of the embodiment is prepared bymixing the photocatalytic composite microparticles with the aqueousdispersion medium and subjecting the resultant to a dispersion processby an ultrasonic dispersion machine, a wet jet mill, a bead mill, or thelike. In such an aqueous dispersion, the photocatalytic compositemicroparticles contain agglomerated particles in which the primaryparticles are aggregated. When the D50 particle size in a volume-basedintegrated diameter is in the range of not less than 20 nm nor more than10 μm as a result of measuring the particle size distribution inclusiveof the agglomerated particles by a wet laser diffraction particle sizedistribution analyzer, it is possible to obtain a good dispersion stateand uniform and stable film formability of the photocatalytic compositemicroparticles. As a result, high photocatalytic performance can beexhibited.

In order to form a stable aqueous dispersion and obtain a uniformphotocatalytic composite microparticle film by using this aqueousdispersion, the smaller the D50 particle size of the compositemicroparticles is the more preferable. When the D50 particle size of thecomposite microparticles is over 10 it is not possible to obtain asufficient property as the aqueous dispersion including thephotocatalytic composite microparticles. On the other hand, when the D50particle size of the composite microparticles is smaller than 20 nm,handleability of the raw material powder decreases because the particlesare too small, leading to low practicality of the raw material powderand the aqueous dispersion prepared by using this. The D50 particle sizeof the photocatalytic composite microparticles is preferably in a rangeof not less than 50 nm nor more than 1 μm, and more preferably in arange of from 50 to 300 nm.

In the photocatalytic composite microparticles contained in the aqueousdispersion, a D90 particle size in the particle size distribution ispreferably in a range of from 0.05 μm to 10 μm. When the D90 particlesize of the composite microparticles is less than 0.05 μm,dispersibility deteriorates because of too small a particle size of thewhole photocatalytic composite microparticles. This makes it difficultto obtain a uniform dispersion and a uniform coating material. When theD90 particle size of the composite microparticles is over 10 urn, filmformability of the aqueous dispersion deteriorates, which makes itdifficult to form a uniform and stable film. Accordingly, it may not bepossible for the photocatalytic performance to be fully exhibited.

In order to form a uniform and smooth film and a high-strength film bythe aqueous dispersion of the present invention, it is preferable tomake the D90 particle size as small as possible by disintegrating theagglomerated particles. In order for the photocatalytic performance tobe exhibited after the photocatalytic composite microparticles includingthe tungsten oxide according to the present invention is formed into afilm, a condition is preferably set so that the dispersion process doesnot give an excessive strain to the microparticles. In order to form auniform and stable film by using an aqueous dispersion and a coatingmaterial having good dispersibility, it is preferable to apply theaqueous dispersion or the coating material by a method such as spincoating, dipping, or spraying.

The performance of the photocatalytic microparticles is generally higheras a specific surface area is larger and the particle size is smaller.The tungsten oxide used in the photocatalytic composite microparticlesis preferably tungsten oxide microparticles whose average primaryparticle size (D50 particle size) is in a range of from 1 to 400 nm. TheBET specific surface area of the tungsten oxide microparticles ispreferably in a range of from 4.1 to 820 m²/g. When the average primaryparticle size of the tungsten oxide microparticles is over 400 nm orwhen the BET specific surface area thereof is less than 4.1 m²/g, thephotocatalytic performance of the tungsten oxide microparticlesdeteriorates and it becomes difficult to form a uniform and stable film.When the average primary particle size of the tungsten oxidemicroparticles is too small, dispersibility also deteriorates and itbecomes difficult to prepare a uniform dispersion. The average primaryparticle size of the tungsten oxide microparticles is more preferably ina range of from 2.7 to 75 nm, and still more preferably in a range offrom 5.5 to 51 nm. The BET specific surface area of the tungsten oxidemicroparticles is more preferably in a range of from 11 to 300 m²/g, andstill more preferably in a range of from 16 to 150 m²/g.

Further, a crystal structure of the tungsten oxide microparticles ispreferably stable. An unstable crystal structure is liable todeteriorate a dispersion state since, when the aqueous dispersion isstored for a long period, the crystal structure of the tungsten oxidemicroparticles changes and accordingly liquidity changes. Further, thetungsten oxide microparticles may contain a minute amount of a metalelement and so on as impurities. The content of the metal element as animpurity element is preferably 2 mass % or less. The impurity metalelements include an element generally contained in a tungsten ore, apollutant element which is mixed when a tungsten compound or the likeused as the raw material is manufactured, and so on, and examplesthereof are Fe, Mo, Mn, Cu, Ti, Al, Ca, Ni, Cr, Mg, and the like. Thisdoes not apply when these elements are used as constituent elements ofthe composite material.

When the tungsten oxide microparticles and the zirconium oxidemicroparticles are compounded, a ratio of an average primary particlessize of the zirconium oxide microparticles (D50_(ZrO2)) to the averageprimary particle size of the tungsten oxide microparticles (D50_(WO3))is preferably in a range of from 0.05 to 20. When the ratio of theaverage primary particle sizes (D50_(ZrO2)/D50_(WO3)) is smaller than0.05 or larger than 20, primary particles of the tungsten oxide and theprimary particles of the zirconium oxide extremely differ in size, whichis likely to deteriorate uniform dispersibility of the compositemicroparticles in the aqueous dispersion medium. Accordingly, the effectof improving the photocatalytic performance of the tungsten oxide basedon the gas adsorption power of the zirconium oxide decreases. Aspreviously described, the zirconium oxide preferably has the rod-shapedprimary particles. In this case, the average primary particles size ofthe zirconium oxide microparticles refers to an average major axis ofthe rod-shaped particles. The ratio of the average primary particlesizes (D50_(ZrO2)/D50_(WO3)) is more preferably in a range of from 0.1to 5.

The aqueous dispersion of the embodiment has pH in the range of from 1to 9. Since, when pH of the aqueous dispersion containing thevisible-light responsive photocatalytic composite microparticles fallsin the range of from 1 to 9, a zeta potential of the aqueous dispersionbecomes minus, it is possible to realize an excellent dispersion state.According to such a dispersion and a coating material using the same,the thin and uniform coating of the base material or the like ispossible. pH of the aqueous dispersion has a correlation with aconcentration of the photocatalytic composite microparticles (particleconcentration in the aqueous dispersion), and when pH changes, thedispersion state changes. When pH is in the range of from 1 to 9, a gooddispersion state can be obtained.

When pH of the aqueous dispersion is less than 1, the zeta potentialapproaches zero, so that dispersibility of the photocatalytic compositemicroparticles deteriorates. When pH of the aqueous dispersion is morethan 9, pH the aqueous dispersion approaches an alkali side too much, sothat the tungsten oxide is easily dissolved. In order to adjust pH ofthe aqueous dispersion, an acid or alkali aqueous solution ofhydrochloric acid, sulfuric acid, tetramethylammonium hydroxide (TMAH),ammonia, sodium hydroxide, or the like may be added.

pH of the aqueous dispersion is preferably in a range of from 2.5 to7.5. By setting pH of the aqueous dispersion in the range of from 2.5 to7.5, the photocatalytic performance (gas decomposition performance) canbe more effectively exhibited. When a surface state of the particles isobserved by FT-IR (Fourier transform infrared absorption spectroscopy)after an aqueous dispersion whose pH is in the range of from 2.5 to 7.5is applied and dried, the absorption or a hydroxyl group is seen near3700 cm⁻¹. By using such a film as a photocatalytic film, it is possibleto obtain excellent decomposition performance for organic gas. When anaqueous dispersion whose pH is adjusted to 8 is applied and dried, theabsorption of the hydroxyl group is reduced and gas decompositionperformance also deteriorates. When pH of the aqueous dispersion isadjusted to 1.5, though the hydroxyl group exists, the zeta potentialapproaches 0, so that dispersibility slightly deteriorates and gasdecomposition performance also slightly deteriorates.

Further, the aqueous dispersion preferably has a color whose a* is 10 orless, b* is −5 or more, and L* is 50 or more when the color of theaqueous dispersion is expressed by the L*a*b* color system. By applyinga dispersion having such a color tone on a base material to form a film,it is possible to obtain good photocatalytic performance, and inaddition, color of the base material is not impaired. Therefore, it ispossible to obtain a coating material and a film.

The concentration of the photocatalytic composite microparticles(particle concentration) in the aqueous dispersion of the embodiment ispreferably in a range of not less than 0.001 mass % nor more than 50mass %. When the particle concentration is less than 0.001 mass %, thecontent of the photocatalytic composite microparticles becomesinsufficient, and it may not be possible to obtain desired performance.When the particle concentration is over 50 mass %, the photocatalyticcomposite microparticles exist in close contact when they are formedinto a film, and it is not possible to obtain a particle surface arealarge enough to make their performance exhibited. Not only sufficientperformance cannot be exhibited but also cost of the dispersion and thefilm increases since a more than necessary amount of the photocatalyticcomposite microparticles are contained.

The concentration of the photocatalytic composite microparticles is morepreferably in a range of from 0.01 to 20 mass %. According to thedispersion in which the concentration of the photocatalytic compositemicroparticles is 20 mass % or less, it is possible to easily realize astate in which the photocatalytic composite microparticles are uniformlydispersed. In a high-concentration dispersion containing thephotocatalytic composite microparticles whose concentration is over 20mass %, the dispersion state of the photocatalytic compositemicroparticles improves when the dispersion is diluted to prepare acoating material. Therefore, the use of the high-concentrationdispersion containing 20 mass % photocatalytic composite microparticlesor more has an advantage that the coating material in which thephotocatalytic composite microparticles are uniformly dispersed can beefficiently prepared.

The photocatalytic composite microparticles contained in the aqueousdispersion may contain not only the tungsten oxide and the zirconiumoxide but also a metal element (hereinafter referred to as an additivemetal element) other than tungsten and zirconium. Examples of the metalelement contained in the photocatalyst are a transition metal elementexcept tungsten and zirconium, a zinc group element such as zinc, and anearth metal element such as aluminum. The transition metal elements aremetals with atomic numbers 21 to 29, 39 to 47, 57 to 79, and 89 to 109,and among them, the metal elements except tungsten and zirconium can becontained in the photocatalytic composite microparticles. The zinc groupelements are elements with atomic numbers 30, 48, 80, and the earthmetal elements are elements with atomic numbers 13, 31, 49, 81. Thesemetal elements may be contained in the photocatalytic compositemicroparticles. Adding these metal elements to the photocatalyticcomposite microparticles makes it possible to improve performance of thephotocatalytic composite microparticles.

The content of the additive metal element in the photocatalyticcomposite microparticles is preferably in a range of from 0.001 to 50mass % to the tungsten oxide. When the content of the additive metalelement is over 50 mass % to the tungsten oxide, a property based on thetungsten oxide microparticles is liable to deteriorate. The content ofthe additive metal element is more preferably 10 mass % or less to thetungsten oxide. A lower limit value of the content of the additive metalelement is not particularly limited, but in order to make the effect ofadding the metal element more effectively exhibited, its content ispreferably 0.001 mass % or more to the tungsten oxide. In order toprevent dispersibility deterioration of the aqueous dispersion, thecontent and a form of the additive metal element are preferably adjustedso as not to cause a great change in pH and the zeta potential. Inconsideration of such points, the content of the additive metal elementis more preferably 2 mass % or less to the tungsten oxide.

The metal element (additive metal element) contained in thephotocatalyst is preferably at least one selected from nickel (Ni),titanium (Ti), manganese (Mn), iron (Fe), palladium (Pd), platinum (Pt),ruthenium (Ru), copper (Cu), silver (Ag), aluminum (Al), and cerium(Ce). By making these metal elements contained in the photocatalyst in arange of from 0.005 to 10 mass %, it is possible to more effectivelyimprove the photocatalytic performance of the photocatalyst of theembodiment. The content of the above metal element is more preferably ina range of from 0.005 to 2 mass % to the tungsten oxide.

In the photocatalytic composite microparticles of the embodiment, themetal element can exist in various forms. The photocatalytic compositemicroparticles can contain the metal element as an element substance ofthe metal element, a compound such as an oxide of the metal element, acomplex compound with tungsten oxide or zirconium oxide, or the like.The metal element contained in the photocatalytic compositemicroparticles themselves may form a compound with another element. Atypical form of the metal element in the photocatalytic compositemicroparticles is an oxide of the metal element. The metal element ismixed in the form of an element substance, a compound, a complexcompound, or the like with, for example, the tungsten oxide powder orthe zirconium oxide powder.

Further, a method of compounding the metal element in the photocatalyticcomposite microparticles is not particularly limited. As the compoundingmethod of the metal element, a mixing method of mixing powders, animmersion method, a carrying method, or the like can be employed.Typical compounding methods are described below. An example of a methodof compounding Ru is a method of adding a ruthenium chloride solution tothe dispersion containing tungsten oxide and zirconium oxide. An exampleof a method of compounding Pt is a method of mixing a Pt powder to thedispersion containing tungsten oxide and zirconium oxide. An example ofa method of compounding Cu is a method in which the tungsten oxidemicroparticles and the zirconium oxide microparticles are added andmixed in a water solution or an ethanol solution of copper nitrate orcopper sulfate, followed by drying at a temperature of 70 to 80° C. andfiring at a temperature of 500 to 550° C. (immersion method).

In the compounding method of the metal element, the immersion method isalso applicable to an iron compounding method using an iron chloridesolution, a silver compounding method using a silver chloride solution,a platinum compounding method using a chloroplatinic acid solution, apalladium compounding method using a palladium chloride solution, andthe like. Further, tungsten oxide and zirconium oxide may be compoundedwith the metal element (oxide) by using oxide sol such as titanium oxidesol, aluminum sol, or the like.

In the aqueous dispersion of the embodiment, the aqueous dispersionmedium is preferably water. However, as a dispersion medium other thanwater, alcohol may be contained in a range of less than 50 mass %.Methanol, ethanol, 1-propanol, 2-propanol, or the like is used as thealcohol. When the content of the alcohol is over 50 mass %, theaggregation is facilitated. The content of the alcohol is morepreferably 20 mass % or less, and still more preferably 10 mass % orless. In the aqueous dispersion of the embodiment, the photocatalyticcomposite microparticles may be dispersed in the aqueous dispersionmedium while being mixed with, carried by, or immersed in a materialhaving adsorption performance such as activated carbon or zeolite.

The tungsten oxide microparticles (powder) used in the aqueousdispersion of the embodiment are preferably prepared by the followingmethods, but the method is not limited to these. The tungsten oxidemicroparticles are preferably prepared by employing a sublimationprocess. Combining a heat treatment process with the sublimation processis also effective. According to tungsten trioxide microparticlesprepared by such a method, it is possible to stably realize theaforesaid average primary particle size, BET specific surface area, andcrystal structure. Further, the average primary particle sizeapproximates a value converted from the BET specific surface area, andit is possible to stably obtain microparticles (fine powder) having asmall variation in particle size.

First, the sublimation process will be described. The sublimationprocess is a process of obtaining the tungsten trioxide microparticlesby sublimating a metal tungsten powder, a tungsten compound powder, or atungsten compound solution in an oxygen atmosphere. The sublimation is aphenomenon that a state change from a solid phase to a vapor phase orfrom a vapor phase to a solid phase occurs without going through aliquid phase. By oxidizing the metal tungsten powder, the tungstencompound powder, or the tungsten compound solution as the raw materialwhile sublimating it, it is possible to obtain a tungsten oxide powderin a microparticle state.

As the raw material (tungsten raw material) of the sublimation process,any of the metal tungsten powder, the tungsten compound powder, and thetungsten compound solution may be used. Examples of the tungstencompound used as the raw material are tungsten oxide such as tungstentrioxide (WO₃), tungsten dioxide (WO₂), and a low-grade oxide, tungstencarbide, ammonium tungstate, calcium tungstate, tungstic acid, and thelike.

When the sublimation process of the tungsten raw material as describedabove is performed in the oxygen atmosphere, the metal tungsten powderor the tungsten compound powder instantaneously changes from the solidphase to the vapor phase, and when metal tungsten vapor in the vaporphase is further oxidized, the tungsten oxide microparticles areobtained. When a solution is used as well, the tungsten raw materialchanges to the vapor phase through a tungsten oxide or compound. By thususing an oxidative reaction in the vapor phase, it is possible to obtainthe tungsten oxide microparticles. Further, it is possible to controlthe crystal structure of the tungsten oxide microparticles.

As the raw material of the sublimation process, at least one kindselected from a metal tungsten powder, a tungsten oxide power, atungsten carbide powder, and an ammonium tungstate powder is preferablyused because impurities are not easily contained in the tungsten oxidemicroparticles obtained after the sublimation in the oxygen atmosphere.Not containing a harmful substance as a by-product (substance other thantungsten oxide) formed by the sublimation process, the metal tungstenpowder and the tungsten oxide powder are especially preferable as theraw material of the sublimation process.

As the tungsten compound used as the raw material, a compound containingtungsten (W) and oxygen (O) as its constituent elements is preferable.When W and O are contained as the constituent components, the rawmaterial is easily sublimated instantaneously when later-describedinductively coupled plasma processing or the like is employed in thesublimation process. Examples of such a tungsten compound are WO₃,W₂₀O₅₈, W₁₈O₄₉, WO₂, and the like. Further, a solution, a salt, or thelike of tungstic acid, ammonium paratungstate, or ammonium metatungstateis also effective.

When the tungsten oxide composite microparticles are prepared, atransition metal element and other element may be mixed in a form ofmetal, a compound containing an oxide, a complex compound, or the likein addition to the tungsten raw material. By processing the tungstenoxide simultaneously with the other element, it is possible to obtaincomplex compound microparticles such as a composite oxide of thetungsten oxide and the other element. It is also possible to obtain thetungsten oxide composite microparticles by mixing or carrying thetungsten oxide microparticles with/by element particles or compoundparticles of the other element. A compounding method of the tungstenoxide and the other metal element is not particularly limited, andvarious well-known methods can employed.

The metal tungsten powder or the tungsten compound powder as thetungsten raw material preferably has an average particle size in a rangeof from 0.1 to 100 μm. The average particle size of the tungsten rawmaterial is more preferably in a range of from 0.3 μm to 10 μm, and morepreferably in a range of from 0.3 μm to 3 μm. The use of the metaltungsten powder or the tungsten compound powder having the averageparticle size within the aforesaid range easily causes the sublimation.When the average particle size of the tungsten raw material is less than0.1 μM, advance adjustment of the raw material powder is necessary orhandleability decreases because the raw material powder is too fine.When the average particle size of the tungsten raw material is over 100μm, a uniform sublimation reaction is difficult to occur. Even when theaverage particle size is large, a uniform sublimation reaction can becaused by the processing with a large energy amount, but this is notindustrially preferable.

As a method to sublimate the tungsten raw material in the oxygenatmosphere in the sublimation process, usable is at least one kind ofprocessing selected from inductively coupled plasma processing, arcdischarge processing, laser processing, electron beam processing, andgas burner processing. Among them, the laser processing and the electronbeam processing perform the sublimation process by laser or electronbeam irradiation. The laser and the electron beam has a smallirradiation spot diameter, and therefore has a merit that the particlesize of the raw material powder and stability of its supply amount neednot be strictly controlled though taking a long time to process a largeamount of the raw material at a time.

The inductively coupled plasma processing and the are dischargeprocessing can cause the oxidative reaction of a large amount of the rawmaterial powder in the oxygen atmosphere at a time though requiringadjustment of a generation region of plasma and arc discharge. Further,it is possible to control an amount of the raw material that can beprocessed at a time. The gas burner processing has a difficulty inprocessing a large amount of the raw material powder and the rawmaterial solution though requiring a relatively low power expense.Therefore, the gas burner processing is inferior in productivity.Incidentally, the gas burner processing may be any provided that it hasenergy large enough to cause the sublimation, and is not particularlylimited. A propane gas burner, an acetylene gas burner, and the like areused.

When the inductively coupled plasma processing is employed for thesublimation process, generally used is a method in which argon gas andoxygen gas are used to generate plasma and the metal tungsten powder orthe tungsten compound powder is supplied into the plasma. Examples of amethod of supplying the tungsten raw material into the plasma are amethod of blowing the metal tungsten powder or the tungsten compoundpowder together with carrier gas, a method of blowing a dispersion inwhich the metal tungsten powder or the tungsten compound powder isdispersed in a predetermined liquid dispersion medium, and so on.

Examples of the carrier gas used when the metal tungsten powder or thetungsten compound powder is blown into the plasma are air, oxygen, inertgas containing oxygen, and so on. Among them, the air is preferably usedbecause of its low cost. Inert gas such as argon and helium may be usedas the carrier gas when a reaction field contains sufficient oxygen suchas when reaction gas containing oxygen besides the carrier gas issupplied or when the tungsten compound powder is tungsten trioxide.Oxygen, inert gas containing oxygen, or the like is preferably used asthe reaction gas. When the inert gas containing oxygen is used, it ispreferable that an amount of oxygen is set so as to enable the supply ofa sufficient amount of oxygen necessary for the oxidative reaction.

By employing the method of blowing the metal tungsten powder or thetungsten compound powder together with the carrier gas and by adjustinga gas flow rate, a pressure of a reaction vessel, and so on, it becomeseasy to control the crystal structure of the tungsten trioxidemicroparticles. Concretely, it is easy to obtain at least one kindselected from a monoclinic crystal and a triclinic crystal (monocliniccrystal, triclinic crystal, or mixed crystal of monoclinic crystal andtriclinic crystal) or to obtain the tungsten trioxide microparticleshaving the crystal structure in which a rhombic crystal is mixed in theabove. The crystal structure of the tungsten trioxide microparticles ismore preferably a mixed crystal of a monoclinic crystal and a tricliniccrystal or a mixed crystal of a monoclinic crystal, a triclinic crystal,and a rhombic crystal.

An example of the dispersion medium used for preparing the dispersion ofthe metal tungsten powder or the tungsten compound powder is a liquiddispersion medium having oxygen atoms in its molecules. The use of thedispersion facilitates handling the raw material powder. As the liquiddispersion medium having the oxygen atoms in its molecules, onecontaining 20 vol % or more of at least one kind selected from water andalcohol is used, for instance. As the alcohol used as the liquiddispersion medium, at least one kind selected from methanol, ethanol,1-propanol, and 2-propanol is preferable, for instance. Water andalcohol are easily volatilized by heat of the plasma and thus do notobstruct the sublimation reaction and the oxidative reaction of the rawmaterial powder, and they contain oxygen in their molecules and thuseasily promote the oxidative reaction.

When the dispersion is prepared by dispersing the metal tungsten powderor the tungsten compound powder in the dispersion medium, the content ofthe metal tungsten powder or the tungsten compound powder in thedispersion is preferably in a range of from 10 to 95 mass %, and morepreferably in a range of from 40 to 80 mass %. By dispersing the metaltungsten powder or the tungsten compound powder with such a contentrange in the dispersion, it is possible to uniformly disperse the metaltungsten power or the tungsten compound powder in the dispersion. Theuniformly dispersed state facilitates the uniform sublimation reactionof the raw material powder. When the content in the dispersion is lessthan 10 mass %, efficient manufacture is not enabled due to too small anamount of the raw material powder. When the content is over 95 mass %,an amount of the dispersion is small and hence the viscosity of the rawmaterial powder increases, so that the raw material powder is likely tostick to the vessel, which decreases handleability.

By employing the method of blowing the metal tungsten powder or thetungsten compound powder as the dispersion into the plasma, it becomeseasy to control the crystal structure of the tungsten trioxidemicroparticles. Concretely, it is easy to obtain the tungsten trioxidemicroparticles having the crystal structure in which at least one kindselected from the monoclinic crystal and the triclinic crystal is mixedor the rhombic crystal is mixed with this. Further, the use of atungsten compound solution as the raw material also enables a uniformsublimation reaction and in addition improves controllability of thecrystal structure of the tungsten trioxide microparticles. The aforesaidmethod using the dispersion is also applicable to the arc dischargeprocessing.

When the sublimation process is carried out by the irradiation of thelaser or the electron beam, a pelletized metal tungsten or tungstencompound is preferably used as the raw material. Since the laser or theelectron beam has a small irradiation spot diameter, the supply becomesdifficult when the metal tungsten powder or the tungsten compound powderis used, but the use of the pelletized metal tungsten or tungstencompound enables efficient sublimation. The laser may be any, providedthat it has energy large enough to sublimate the metal tungsten or thetungsten compound, and is not particularly limited, but CO₂ laser ispreferable because of its high energy.

Moving at least one of an irradiation source of the laser light or theelectron beam and the pellet when the pellet is irradiated with thelaser or the electron beam makes it possible to effectively sublimatethe whole surface of the pellet having a certain size. Consequently, itis possible to easily obtain the tungsten trioxide powder having thecrystal structure in which at least one kind selected from themonoclinic crystal and the triclinic crystal is mixed with the rhombiccrystal. The aforesaid pellet is also applicable to the inductivelycoupled plasma processing and the arc discharge processing.

The tungsten oxide microparticles used in the aqueous dispersion of thisembodiment can be obtained only by the sublimation process describedabove, but it is also effective to apply a heat-treat process to thetungsten oxide microparticles prepared by the sublimation process. Theheat treatment process is to heat-treat the tungsten trioxidemicroparticles obtained in the sublimation process at a predeterminedtemperature for a predetermined time in an oxygen atmosphere. Even whenthe tungsten trioxide microparticles cannot be sufficiently formed underthe control condition and so on of the sublimation process, the heattreatment can make a ratio of the tungsten trioxide microparticles inthe tungsten oxide microparticles 99% or more, practically 100%.Further, the heat treatment process can adjust the crystal structure ofthe tungsten trioxide microparticles to a predetermined structure.

An example of the oxygen atmosphere used in the heat treatment processis air and oxygen-containing gas. The oxygen-containing gas means inertgas containing oxygen. The heat treatment temperature is preferably in arange of from 200 to 1000° C., and more preferably 400 to 700° C. Theheat treatment time is preferably ten minutes to five hours, and morepreferably thirty minutes to two hours. By setting the temperature andthe time of the heat treatment process to the aforesaid ranges, it iseasy to form tungsten trioxide from tungsten oxide other than tungstentrioxide, Further, in order to obtain a powder having a small number ofdefects and good crystallinity, it is preferable to slowly increase anddecrease the temperature at the time of the heat treatment. Rapidheating and cooling at the time of the heat treatment causedeterioration of crystallinity.

When the heat treatment temperature is lower than 200° C., it may not bepossible to sufficiently obtain an oxidizing effect for turning thepowder which does not change into tungsten trioxide in the sublimationprocess into tungsten trioxide. When the heat treatment temperature ishigher than 1000° C., the tungsten oxide microparticles rapidly grow andaccordingly a specific surface area of the obtained tungsten oxidemicroparticles is likely to decrease. Further, the heat treatmentprocess at and for the aforesaid temperature and time makes it possibleto adjust the crystal structure and crystallinity of the tungstentrioxide microparticles.

The aqueous dispersion of this embodiment is usable as a film formingmaterial as it is. The aqueous dispersion may be mixed with a bindercomponent or the like to fabricate a coating material, and the coatingmaterial may be used as the film forming material. The coating materialcontains not only the aqueous dispersion but also at least one kind of abinder component selected from an inorganic binder and an organicbinder. The content of the binder component is preferably in a range offrom 5 to 95 mass %. When the content of the binder component is over 95mass %, it may not be possible to obtain desired photocatalyticperformance. When the content of the binder component is less than 5mass %, sufficient bonding strength cannot be obtained and a filmproperty is liable to deteriorate. Applying such a coating materialmakes it possible to adjust strength, hardness, adhesion to the basematerial, and so on of the film to desired states.

As the inorganic binder, used are, for example, alkyl silicate, siliconhalide, a product obtained through decomposition of a hydrolytic siliconcompound such as a partial hydrolysate of any of these, anorganopolysiloxane compound and its polycondensate, silica, colloidalsilica, water glass, a silicon compound, phosphate such as zincphosphate, metal oxide such as zinc oxide and zirconium oxide,biphosphate, cement, gypsum, lime, enameling frit, and so on. As theorganic binder, used are, for example, fluorine-based resin, siliconeresin, acrylic resin, epoxy resin, polyether resin, melamine resin,urethane resin, alkyd resin, and so on.

Applying the aforesaid aqueous dispersion or coating material on thebase material makes it possible to stably and uniformly form a filmcontaining the visible-light responsive photocatalytic compositemicroparticles. As the base material forming such a photocatalytic film,used are glass, ceramics, plastic, resin such as acryl, paper, fibermetal, wood, and so on. A film thickness is preferably in a range offrom 2 to 1000 nm. When the film thickness is less than 2 nm, it may notbe possible to obtain a state where the tungsten oxide microparticlesand the zirconium oxide microparticles uniformly exist. When the filmthickness is over 1000 nm, adhesion to the base material lowers. Thefilm thickness is more preferably in a range of from 2 to 400 nm.

The photocatalytic film of this embodiment exhibits photocatalyticperformance under visible light irradiation. Generally, the visiblelight is light whose wavelength is in a 380 to 830 nm range, and islight radiated by using, as a light source, an ordinary lighting such asa white fluorescent light, sunlight, white LED, an electric bulb, ahalogen lamp, and a xenon lamp, and a blue light-emitting diode, a bluelaser, and the like. The photocatalytic film of this embodiment exhibitsphotocatalytic performance under an ordinary indoor environment. Thephotocatalytic performance is an action in which an electron-hole pairis excited per photon as a result of light absorption, the excitedelectron and hole activate a hydroxyl group and acid present on asurface by redox, and organic gas or the like is oxidatively decomposedby activated oxygen species generated by the activation, and is anaction for exhibiting a hydrophilic property, antibacterial/disinfectionperformance, and the like.

The product of this embodiment includes the photocatalytic film formedby using the aforesaid aqueous dispersion or coating material.Concretely, it is a product in which the photocatalytic film is formedby applying the aqueous dispersion or the coating material on thesurface of the base material forming the product. The film formed on thesurface of the base material may contain zeolite, activated carbon,porous ceramics, and like. The aforesaid photocatalytic film and theproduct including the same have characteristics of being excellent indecomposition performance for organic gas such as acetaldehyde andformaldehyde under visible light irradiation, and exhibiting highactivity especially even under low illuminance. The film of thisembodiment exhibits a hydrophilic property in contact angle measurementof water. Further, in evaluation of antibacterial performance forStaphylococcus aureus and colon bacillus under visible lightirradiation, the film exhibits a high antibacterial action.

Concrete examples of the product including the photocatalytic film ofthe embodiment are an air-conditioner an air cleaner, an electric fan, arefrigerator, a microwave oven, a dish washer/drier, a rice cooker, apot, a pan lid, an IH heater, a washing machine, a cleaner, a lightingfixture (lamp, fixture body, shade, and the like), sanitary goods, atoilet stool, a washstand, a mirror, a bathroom (wall, ceiling, floor,and the like), building materials (indoor wall, ceiling material, floor,outer wall, and the like), interior products (curtain, carpet, table,chair, sofa, shelf, bed, bedding, and the like), glass, a sash, ahandrail, a door, a knob, clothes, filters used in home electricappliances, stationery, kitchen utensils, members used in an indoorspace of an automobile, and so on. Being provided with thephotocatalytic film of the embodiment, the product can be givenvisible-light responsive photocatalytic performance.

When a fiber is used as the base material, examples of a fiber materialare synthetic fibers such as polyester, nylon, and acrylic, regeneratedfibers such as rayon, natural fibers such as cotton, wool, and silk, anda combined filament yarn, union cloth, and a blend of these, and so on.The fiber material may be in a loose form. The fiber may take any formsuch as woven fabric, knitted fabric, nonwoven fabric, and the like, andmay be those having ordinary dyeing and printing. When the aqueousdispersion is used for the fiber material, a method of co-using thephotocatalytic composite microparticles with a resin binder and fixingthe resultant to the fiber material is convenient.

As the resin binder, water-soluble, water-dispersible, orsolvent-soluble resin can be used. Concretely, melamine resin, epoxyresin, urethane resin, acrylic resin, fluorine resin, or the like isused, but these are not restrictive. When the aqueous dispersion is usedand the photocatalytic composite microparticles are fixed to the fibermaterial, for example, the aqueous dispersion is mixed with thewater-dispersible or water-soluble resin binder to prepare a resinliquid, and the fiber material is immersed in the resin liquid andthereafter is squeezed by a mangle roll to be dried. By thickening theresin liquid, it is possible to coat one surface of the fiber materialwith the resin liquid by a well-known apparatus such as a knife coater.It is also possible to make the visible-light responsive photocatalyticcomposite microparticles adhere on one surface or both surfaces of thefiber material by using a gravure roll.

When the aqueous dispersion is used and the photocatalytic compositemicroparticles are made to adhere on the fiber surface, if an adhesionamount is too small, it is not possible to sufficiently exhibitphotocatalytic performance such as gas decomposition performance andantibacterial performance of tungsten oxide has. When the adhesionamount is too large, though the performance that tungsten oxide has isexhibited, feeling as the fiber material sometimes deteriorates. Anappropriate adhesion amount is preferably selected according to thematerial or an intended use. Clothes and interior products using thefiber on whose surface the photocatalytic composite microparticlescontained in the aqueous dispersion adhere exhibit excellent odoreliminating effect and antibacterial effect under visible lightirradiation in an indoor environment.

Next, examples and evaluation results thereof will be described. In thefollowing examples, the inductively coupled plasma processing isemployed in the sublimation process, but the present invention is notlimited to this.

Example 1

First, a tungsten trioxide powder having a 0.5 μn average particle sizewas prepared as a raw material powder. This raw material powder wassprayed together with carrier gas (Ar) to RF plasma, and further asreaction gas, argon was supplied at a 40 L/min flow rate and oxygen wassupplied at a 40 L/mn flow rate. In this manner, through the sublimationprocess to cause the raw material powder to undergo an oxidativereaction while sublimating the raw material powder, a tungsten oxidepowder was prepared. The tungsten oxide powder was heat-treated in theatmosphere under a condition of 900° C.×1.5 h. The tungsten oxide powderwas dispersed in water so that its concentration became 10 mass %. Inthis manner, a first dispersion containing tungsten oxide microparticleswas prepared.

An average primary particle size (D50 particle size) and a BET specificsurface area of the obtained tungsten oxide powder were measured. Theaverage primary particle size was measured by image analysis of a TEMphotograph. In the TEM observation, a transmission electron microscopeH-7100FA (trade name, manufactured by Hitachi Ltd.) was used, anenlarged photograph was subjected to the image analysis, 50 pieces ofthe particles or more were extracted, a volume-based integrated diameterwas found, and the D50 particle size was calculated. For the measurementof the BET specific surface area, a specific surface area analyzerMacsorb 1201 (trade name, manufactured by Mountech Co., Ltd.) was used.A pre-process was carried out in nitrogen under a condition of200°×twenty minutes. The average primary particle size (D50 particlesize) was 25 nm and the BET specific surface area was 35 m²/g.

Next, a zirconium oxide powder having rod-shaped primary particles wasprepared. A D50 particle size (average major axis of the primaryparticles) in particle size distribution of the zirconium oxide powderis 20 nm. A ratio of the average major axis of the primary particles(average primary particle size) of the zirconium oxide powder to theaverage primary particle size of the tungsten oxide powder is 0.8. Thezirconium oxide powder was dispersed in water so that its concentrationbecame 20 mass %, whereby a second dispersion was prepared.

The first dispersion and the second dispersion were mixed so that a massratio of zirconium oxide to tungsten oxide became 100%. A ratio ofatomicity of the zirconium oxide to atomicity of the tungsten oxide is150%. pH of a mixed dispersion of the first dispersion and the seconddispersion was adjusted so as to fall in a range of from 5.5 to 6.5 byusing water, sulfuric acid, and TMAH. For the dispersion process, a beadmill was used. In this manner, an intended aqueous dispersion wasprepared. In the aqueous dispersion, a particle concentration(concentration of a mixture of the tungsten oxide microparticles and thezirconium oxide microparticles) was 12 mass % and a pH value was 5.

Example 2

By using a tungsten oxide powder prepared by the same method as that ofthe example 1, a first dispersion was prepared in the same manner asthat of the example 1. When an average primary particle size (D50particle size) of the tungsten oxide powder was measured in the samemanner as that of the example 1, the average primary particle size was25 nm. After the tungsten oxide powder was dispersed in water so thatits concentration became 10 mass %, a ruthenium chloride solution wasmixed so that a ratio of ruthenium to the tungsten oxide became 0.02mass %. pH of this mixed liquid was adjusted to 6 while ammonia wasdropped to this mixed liquid.

Next, a zirconium oxide powder having rod-shaped primary particles wasprepared. A D50 particle size (average major axis of the primaryparticles) in particle size distribution of the zirconium oxide powderis 20 nm. A ratio of the average major axis of the primary particles(average primary particle size) of the zirconium oxide powder to theaverage primary particle size of the tungsten oxide powder is 0.8. Thezirconium oxide powder was dispersed in water so that its concentrationbecame 20 mass %, whereby a second dispersion was prepared.

The second dispersion containing the zirconium oxide microparticles wasdropped to the first dispersion containing the tungsten oxidemicroparticles and ruthenium chloride, and pH was adjusted so as to fallin a range of from 5.5 to 6.5. In an aqueous dispersion, a mass ratio ofzirconium oxide to tungsten oxide is 50%, and a ratio of atomicity ofthe zirconium oxide to atomicity of the tungsten oxide is 75%. In thismanner, an intended aqueous dispersion was prepared. In the aqueousdispersion, a particle concentration was 12 mass % and a pH value was 6.

Example 3

By using a tungsten oxide powder prepared by the same method as that ofthe example 1, a first dispersion was prepared in the same manner asthat of the example 1. When an average primary particle size (D50particle size) of the tungsten oxide powder was measured in the samemanner as that of the example 1, the average primary particle size was25 nm. After the tungsten oxide powder was dispersed in water so thatits concentration became 10 mass %, Pt particles were mixed so that aratio of platinum to tungsten oxide became 2 mass %.

Next, a zirconium oxide powder having rod-shaped primary particles wasprepared. A D50 particle size (average major axis of the primaryparticles) in particle size distribution of the zirconium oxide powderis 20 nm. A ratio of the average major axis of the primary particles(average primary particle size) of the zirconium oxide powder to theaverage primary particle size of the tungsten oxide powder is 0.8. Thezirconium oxide powder was dispersed in water so that its concentrationbecame 20 mass %, whereby a second dispersion was prepared.

The first dispersion containing the tungsten oxide microparticles andthe Pt microparticles and the second dispersion containing the zirconiumoxide microparticles were mixed so that a mass ratio of zirconium oxideto tungsten oxide became 10%. A ratio of atomicity of the zirconiumoxide to atomicity of the tungsten oxide is 15%. pH of a mixeddispersion of the first dispersion and the second dispersion wasadjusted so as to fall in a range of from 5.5 to 6.5 by using water,sulfuric acid, and TMAH. For the dispersion process, a bead mill wasused. In this manner, an intended aqueous dispersion was prepared. Inthe aqueous dispersion, a particle concentration was 12 mass % and a pHvalue was 7.

Comparative Example 1

As an aqueous dispersion of a comparative example 1, an aqueousdispersion in which only the tungsten oxide powder was dispersed in theexample 1 was prepared.

By using the aqueous dispersions of the above-described examples 1 to 3and comparative example 1, photocatalytic films were formed on glasssurfaces. Photocatalytic performance of the photocatalytic films undervisible light irradiation was evaluated. The photocatalytic performancewas evaluated by measuring a decomposition rate of acetaldehyde gas.Concretely, the gas decomposition rate was measured under the followingconditions by using a flow-type apparatus similar to that used fornitrogen oxide removal performance (decomposition ability) evaluation ofJIS-R-1701-1 (2004).

A decomposition test of the acetaldehyde gas was conducted as follows.An initial concentration of acetaldehyde is 10 ppm, a gas flow rate is140 mL/min, and a sample amount is 0.2 g. For the adjustment of samples,they are applied on 5×10 cm glass plates to be dried. A pre-process istwelve-hour irradiation with black light. As a light source, a whitefluorescent light (FL20SS•W/8 manufactured by Toshiba Lighting &Technology Corporation) is used, and wavelengths less than 380 nm is cutby using an ultraviolet cut filter (CLAREX N-169 manufactured by NittoJushi Kogyo Co., Ltd.). Illuminance is adjusted to 250 lx. First.without light irradiation, the end and stabilization of gas adsorptionare waited for. After the stabilization, the light irradiation isstarted. The light irradiation is performed under such a condition, anda gas concentration is measured fifteen minutes later to find the gasdecomposition rate. However, when the gas concentration does not becomestable even after fifteen minutes pass, the concentration measurement iscontinued until it is stabilized.

The gas decomposition rate (%) is defined as a value calculated based on[expression: (A−B)/A×100] from a gas concentration A and a gasconcentration B, where A is a gas concentration before the lightirradiation and B is a gas concentration when fifteen minutes or morepass from the light irradiation and the gas concentration is stabilized.As a gas analysis device, a multi-gas monitor 1412 manufactured by INOVAwas used. The measurement results are presented in Table 3. Further,changes of the gas decomposition rate with the elapsed time of the lightirradiation are presented in FIG. 1. Incidentally, properties of rawmaterial powders used to prepare the aqueous dispersions are presentedin Table 1, and properties of the aqueous dispersions are presented inTable 2.

TABLE 1 Raw material powder WO₃ powder ZrO₂ powder Particle D50 particlesize D50 particle size ratio [nm] size [nm] (ZrO₂/WO₃) Example 1 25 200.8 Example 2 25 20 0.8 Example 3 25 20 0.8 Comparative Example 1 25 — —

TABLE 2 Aqueous dispersion Additive ZrO₂/WO₃ Zr/W particle addition massatomicity concentration additive addition ratio* ratio [%] ratio [mass%] pH element form [mass %] Example 1 100 150 12 5 — — — Example 2 50 7512 6 Ru RuCl₃ 0.02 Example 3 10 15 12 7 Pt Pt 2   particles ComparativeExample 1 — — 10 6 — — — *mass ratio of additive element to WO₃

TABLE 3 Gas decomposition rate [%] Example 1 24.3 Example 2 27.8 Example3 24.7 Comparative Example 1 14.1

It was confirmed that photocatalytic films formed by using the aqueousdispersions of the examples 1 to 3 were high in decomposition speed ofacetaldehyde and high in gas decomposition rate. This is becausezirconium oxide adsorbs the gas, so that photocatalytic performance oftungsten oxide was fully exhibited even under an environment with a lowgas concentration. Further, it is seen that photocatalytic performanceis further improved since decomposition performance of an intermediategenerated at the time of the decomposition of the gas also improves.

Further, the aqueous dispersions of the examples 1 to 3 and thecomparative example 1 were each mixed in an acrylic resin-based resinliquid, and a plain weave fabric made of polyester with a 150 g/m² arealweight was immersed in this mixed liquid (coating material), wherebypolyester fibers on which the photocatalytic composite microparticlesadhered were fabricated. 5×10 cm samples were cut out from therespective fibers, and their photocatalytic performance under visiblelight irradiation was evaluated by the same method as the previouslydescribed method. As a result, it was confirmed that the polyesterfibers on which the photocatalytic composite microparticles of theexamples 1 to 3 adhered were higher in the decomposition rate of theacetaldehyde gas than the fiber immersed in the coating material usingthe aqueous dispersion prepared in the comparative example 1. Further,when ten samples fabricated in the same manner were prepared and theirvariation in performance was evaluated, it was confirmed that, since thedispersions of the examples 1 to 3 had excellent dispersibility, anadhesion amount of their photocatalytic composite microparticles to thefibers was stable. Further, it was confirmed that the polyester fiberskept uniform feeling.

According to the photocatalytic films using the aqueous dispersions ofthe examples, it is possible to stably exhibit decomposition performanceof organic gas such as acetaldehyde gas under visible light irradiation.Therefore, they are suitably used for members used in indoor space of anautomobile, building materials used in factories, shops, schools, publicfacilities, hospitals, welfare facilities, accommodation facilities,houses, and so on, interior materials, household electric appliances,and so on. Further, the gas decomposition speed of the photocatalyticfilms do not decrease even when the gas concentration decreases, andhigh gas decomposition performance is maintained. Therefore, it ispossible to obtain high odor eliminating and deodorizing effects. Such aphotocatalytic film and a product using the same can have variousapplications by utilizing the properties that the photocatalyticcomposite microparticles have.

Several embodiments of the present invention are described, but itshould be noted that these embodiments are only exemplary presentationsand are not intended to limit the scope of the invention. These novelembodiments can be implemented in other various forms, and variousomissions, substitutions, and changes can be made therein withoutdeparting from the spirit of the invention. These embodiments and theirmodifications are included in the scope and spirit of the invention andare also included in the scope of the inventions described in the claimsand their equivalencies.

What is claimed is:
 1. An aqueous dispersion, comprising: visible-lightresponsive photocatalytic composite microparticles containing tungstenoxide and zirconium oxide; and an aqueous dispersion medium in which thephotocatalytic composite microparticles are dispersed, wherein a ratioof a mass of the zirconium oxide to a mass of the tungsten oxide in thephotocatalytic composite microparticles is in a range of from 0.05% to200%; wherein a D50 particle size in particle size distribution of thephotocatalytic composite microparticles is in a range of from 20 nm to10 μm; and wherein pH of the aqueous dispersion is in a range of from 1to
 9. 2. The aqueous dispersion according to claim 1, wherein the ratioof the mass of the zirconium oxide to the mass of the tungsten oxide isin a range of from 0.1% to 150%.
 3. The aqueous dispersion according toclaim 1, wherein the photocatalytic composite microparticles containmicroparticles of the tungsten oxide and microparticles of the zirconiumoxide, and wherein a ratio of an average primary particle size of thezirconium oxide microparticles to an average primary particle size ofthe tungsten oxide microparticles is in a range of from 0.05 to
 20. 4.The aqueous dispersion according to claim 3, wherein the zirconium oxidemicroparticles have rod-shaped primary particles, and a ratio of anaverage major axis of the rod-shaped primary particles to the averageprimary particle size of the tungsten oxide microparticles is in a rangeof from 0.05 to
 20. 5. The aqueous dispersion according to claim 1,wherein a ratio of atomicity of zirconium to atomicity of tungsten inthe photocatalytic composite microparticles is in a range of from 0.05%to 400%.
 6. The aqueous dispersion according to claim 1, wherein thephotocatalytic composite microparticles are dispersed in the aqueousdispersion medium in a range of from 0.001 mass % to 50 mass %.
 7. Theaqueous dispersion according to claim 1, wherein the photocatalyticcomposite microparticles contain a metal element other than tungsten andzirconium in a range of from 0.001 mass % to 50 mass % to the tungstenoxide.
 8. The aqueous dispersion according to claim 7, wherein the metalelement is a transition element other than tungsten and zirconium; andwherein the photocatalytic composite microparticles contain the metalelement in a range of from 0.005 mass % to 2 mass % to the tungstenoxide.
 9. The aqueous dispersion according to claim 7, wherein the metalelement is at least one element selected from the group consisting ofnickel, titanium, manganese, iron, palladium, platinum, ruthenium,copper, silver, aluminum, and cerium; and wherein the photocatalyticcomposite microparticles contain the metal element in a range of from0.005 mass % to 2 mass % to the tungsten oxide.
 10. The aqueousdispersion according to claim 7, wherein the metal element is containedin the photocatalytic composite microparticles in at least one formselected from the group consisting of an element substance of the metalelement, a compound of the metal element, and a complex compound of themetal element and tungsten or zirconium.
 11. An aqueous dispersion,comprising; visible-light responsive photocatalytic compositemicroparticles containing tungsten oxide and zirconium oxide; and anaqueous dispersion medium in which the photocatalytic compositemicroparticles are dispersed, wherein a ratio of atomicity of zirconiumto atomicity of tungsten in the photocatalytic composite microparticlesis in a range of from 0.05% to 400%, wherein a D50 particle size inparticle size distribution of the photocatalytic compositemicroparticles is in a range of from 20 nm to 10 μm, and wherein pH ofthe aqueous dispersion is in a range of from 1 to
 9. 12. The aqueousdispersion according to claim 11, wherein the photocatalytic compositemicroparticles contain microparticles of the tungsten oxide andmicroparticles of the zirconium oxide, and wherein a ratio of an averageprimary particle size of the zirconium oxide microparticles to anaverage primary particle size of the tungsten oxide microparticles is ina range of from 0.05 to
 20. 13. The aqueous dispersion according toclaim 12, wherein the zirconium oxide microparticles have rod-shapedprimary particles, and a ratio of an average major axis of therod-shaped primary particles to the average primary particle size of thetungsten oxide microparticles is in a range of from 0.05 to
 20. 14. Theaqueous dispersion according to claim 11, wherein the photocatalyticcomposite microparticles are dispersed in the aqueous dispersion mediumin a range of from 0.001 mass % to 50 mass %.
 15. The aqueous dispersionaccording to claim 11, wherein the photocatalytic compositemicroparticles contain a metal element other than tungsten and zirconiumin a range of from 0.001 mass % to 50 mass % to the tungsten oxide. 16.A coating material, comprising: the aqueous dispersion according toclaim 1; and at least one binder component selected from the groupconsisting of an inorganic binder and an organic binder.
 17. Aphotocatalytic film, configured to be formed by applying the aqueousdispersion according to claim 1 on a base material.
 18. A photocatalyticfilm, configured to be formed by applying the coating material accordingto claim 16 on a base material.
 19. A product comprising thephotocatalytic film according to claim
 17. 20. A product comprising thephotocatalytic film according to claim 18.